Cation channels in basolateral membranes of sheep parotid secretory cells

We observed 240-pS K+ channels in 63% of cell-attached patches, and 30-pS K+ channels were observed in 95% of cell-attached patches. The 240-pS K+ channel had the relative permeability sequence of K+ (1) = Rb+ (1) > Cs+ (0.3) >> Na+ (0.03) and the relative conductance sequence of K+ (1) > Rb+ (0.22) > Cs+ (0.05) > Na+ (0). It was activated by intracellular free Ca2+ and by depolarization. It was blocked by 10 mmol/l tetraethylammonium (TEA) applied extracellularly. The 30-pS K+ channel had the relative permeability sequence of K+ (1) = Rb+ (1) > Cs+ (> Na+ (< 0.09) and the relative conductance sequence of K+ (1) > Rb+ (0.45) > Cs+ (0) = Na+ (0). Its activity was not sensitive to cytosolic free Ca2+ or membrane potential, and it was not blocked by 10 mmol/l TEA extracellularly. Acetylcholine (10 mumol/l) activated the 240-pS voltage-activated and Ca(2+)-activated K+ channels but did not activate the 30-pS K+ channels. We conclude that the 30-pS K+ channel probably determines the properties of the basolateral membrane in unstimulated sheep parotid secretory cells, whereas the 240-pS voltage-activated and Ca(2+)-activated K+ channel may be important during parasympathomimetic stimulation.